References with constant type are pure
References with constant type are pure

Before the fix to #8058, a reference `x` with constant type `0` was
deemed to have a stable member symbol since its symbol did not exist.
That's what gave the reference the `PurePath` status. Now that
`isStableMember` is fixed, this needs to be asserted
explicitly.

lampepfl/dotty,som-snytt/dotty,sjrd/dotty,lampepfl/dotty,som-snytt/dotty,lampepfl/dotty,sjrd/dotty,dotty-staging/dotty,lampepfl/dotty,sjrd/dotty,dotty-staging/dotty,sjrd/dotty,dotty-staging/dotty,lampepfl/dotty,dotty-staging/dotty,dotty-staging/dotty,som-snytt/dotty,som-snytt/dotty,som-snytt/dotty,sjrd/dotty